1. A euphemism combined with a metaphor: Knob: euphemism for dick head, penis (cf. "get my knob polished"). Slobber: visible pre-cum (pre-ejaculate fluid) emanating from the penis, so called because the fluid is clear and visually resembles drool or spittle.

2. By extension, knob slobber can also mean a dick head bearing visible saliva (spittle) from fellatio, together with pre-ejaculate. The combination of the two is also clear.